Stress fractures emanating from re-entrant concrete corners (i.e. inside corners) of a cured or dry concrete structure present serious construction problems to the building industry. Such stress cracks occur as shrinkage stresses develop at the re-entrant corners of the concrete slab. As the magnitude of the transformation from a wet to a cured or dry concrete structure, the stress concentration at the re-entrant corners within a concrete slab dramatically increases which results in a high incidence of cracks and fracturing emanating from the re-entrant corners. These cracks are not only unsightly but can result in substantial structural damage. Unfortunately, corrective reconstruction of structural damaging stress fractures may no only be monetarily costly but also costly in time and effort.
To reduce stress cracks at the re-entrant corners of a concrete slab, concrete finishers have chipped the re-entrant corners of the concrete slab in an effort to reduce the sharp edges of the re-entrant corners. While the corner chipping has sporadically reduced or minimized the formation of re-entrant corners stress cracks, such chipping is time consuming and has not proven entirely satisfactory in substantially eliminating the formation of cracks at the re-entrant corners of a concrete slab.
Certain externally disposed re-entrant corner stress reducing devices have heretofore been unsuccessfully proposed to overcome the fracturing of concrete structures at the re-entrant concrete corners. In U.S. Pat. No. 3,254,463 to Moseley, the Moseley patentee was confronted with the problem of how to initially cast concrete wall sections poured in a horizontal position and when they had adequately cured or dried, the subsequent problem of how to vertically reset the casted concrete wall to its vertical wall position without extensively damaging the upended wall by reason of excessive re-entrant corner cracking such as illustrated by FIG. 2 of the Moseley patent.
To eradicate this problem, Moseley proposed to eliminate or replace the casted and cured concrete re-entrant corner with a semi-cylindrical corner insert to purportedly minimize the cracking tendency of the re-entrant corner when subjecting the corner to considerable stress reducing forces such as uplifting a horizontal positioned wall to the standing wall position. Although this technique was purportedly useful in providing prefabricated walls, the creation of a cylindrical cavity at the re-entrant corners was further compounded by requiring corner extension inserts so as to provide the desired square corner structure at the re-entrant corner, all of which added labor and material costs to the construction as well as detracting from the structural utility and strength of the conventional concrete re-entrant corners of a unititary construction.
Another U.S. patent to Crews et al (U.S. Pat. No. 5,623,798) relies upon an externally disposed stress reducing device to reduce stress cracks caused by concrete shrinkage stresses at the re-entrant corners of the concrete slab. The interfacing surface of the Crews et al device contacting the slab in juxtaposition to the re-entrant concrete corner (referred to as the radius of the arcuate contact engaging surface) is defined by Crew et al as measuring about 1 to about 3 inches in length and extends generally the whole width or depth of the concrete slab. The only specific material mentioned for constructing the right angle shaped brace is STYROFOAM which when imbedded into a re-entrant corner of a concrete slab creates a substantially less durable structure than that of a cured concrete re-entrant corner by itself. Due to its externally disposed position and small size, the Crews et al device fails to provide the overall prerequisites and efficacy as required to overcome the creation of stress fractures and structurally damaging cracks emanating from the re-entrant corners of concrete slabs.